1. Field of the Invention
This is a continuation in part of copending application Ser. No. 09/378,274 filed Aug. 20, 1999. The present invention relates to a vapor generator comprising a high-integrity, high-purity dual chambered chemical vapor deposition (CVD) source vessel. The vapor generator is designed for the unique requirements of vapor phase epitaxy and other chemical vapor deposition equipment used in compound semiconductor and thin-film processes. The vapor generator is designed to safely handle and maintain the highest purity of extremely reactive chemicals.
Specifically, the present invention relates to a dual chambered vapor generator vessel and a method for obtaining steady, i.e. uniform, delivery of a solid organometallic source for chemical vapor deposition systems. A non-uniform delivery rate detrimentally affects the composition of the epitaxial layers grown using a MOVPE (Metal-Organic-Vapor-Phase Epitaxy) system. The present invention also is advantageous when used with MOMBE (Metal-Organic Molecular Beam Epitaxy) and CBE (Chemical Beam Epitaxy) vapor deposition systems.
2. Description of the Prior Art
Group III-V compound semiconductor materials comprising different monocrystalline layers with varying compositions and with thicknesses ranging from fractions of a micron to a few microns are used in the production of many electronic and more particularly optoelectronic devices such as lasers and photodetectors. Chemical vapor deposition methods using organometallic compounds are conventionally employed in the CVD art for the deposition of metal thin-films or semiconductor thin-films of Group III-V compounds. Compounds conventionally used in the semiconductor industry include cyclopentadienyl magnesium (Cp2Mg), trimethyl aluminum (TMA), trimethyl gallium (TMG), triethyl gallium (TEG), trimethyl antimony (TMSb), dimethyl hydrazine (DMHy), trimethyl indium (TMI), etc. Solid precursors, such as TMI are used in the MOVPE of indium containing semiconductors. The solid TMI is placed in a cylindrical vessel or container commonly referred to as a bubbler and subjected to a constant temperature wherein the solid precursor is vaporized. A carrier gas, e.g. hydrogen, is employed to pick up the precursor vapor and transport it to a deposition system. Most solid precursors exhibit poor and erratic delivery rates when used in a conventional bubbler-type precursor vessel.
Conventional bubblers include both bubbler vessels having a dip-tube attached to the inlet exemplified by the disclosure in U.S. Pat. No. 4,506,815 or the gas-feeding device exemplified by the disclosure in U.S. Pat. No. 5,755,885 which has a plurality of gas-ejecting holes in the dip-tube to introduce the carrier gas into the container. These conventional bubbler systems can result in a non-stable, non-uniform flow rate of the precursor vapors especially with solid organometallic precursors. Such non-uniform flow rate produces an adverse affect on the compositions of the semiconductor films being grown in MOVPE reactors. Another bubbler system developed by Morton Intemational, Inc. eliminated the use of a dipxe2x80x94tube. While such dipxe2x80x94tubeless bubbler was found to provide a uniform flow rate, it failed to provide a consistently high concentration of precursor material.
The inability to achieve a stable supply of feed vapor from solid precursors at a consistently high concentration is problematic to the users of such equipment, particularly in the manufacture of semiconductor components. The unsteady organometallic flow rate can be due to a variety of factors: such as progressive reduction in the total surface area of the chemical from which evaporation takes place, channeling through the solid precursor where the carrier gas has minimal contact with the precursor and the sublimation of the precursor solid to parts of the bubbler where efficient contact with the carrier gas is impossible.
Various methods have been adopted to overcome the flow problems such as 1) use of reverse flow bubblers, 2) use of dispersion materials in the precursor materials, 3) employing diffuser plates beneath the bed of solids, 4) employing conical cylinder designs and 5) beating on the cylinder to de-agglomerate the solid precursor. U. S. Pat. No. 4,704,988 discloses a bubbler design wherein the vessel is separated by a porous partition into first and second compartments. Vaporized reactive material contained in the first compartment diffuses through the partition into the second compartment where it contacts and is entrained in a carrier gas for transport from the vessel into the appropriate deposition chamber.
Therefore, as mentioned above the stable flow/pick-up of solid precursor vapor is an ongoing and major drawback associated with prior art bubbler vessels. Present bubbler vessel designs and different delivery configurations fail to provide an uniform flow rate with maximum pick-up of precursor material. Although the single frifted bubbler design of copending application Ser. No. 09/378,274 provides better stability and pickup of precursor vapor than prior art bubblers not employing a frit element and are capable of providing improved precursor pickup, there is still a need in the art for bubbler devices that are tailored to provide a uniform and high concentration of the precursor vapors until total depletion of the vapor source. Another bubbler designed to improve precursor pickup is disclosed in U.S. Pat. No. 5,603,169 and employs lower and upper porous plates through which the carrier gas passes. The lower porous plate is located above the carrier gas feed inlet and supports the solid precursor load. In operation, carrier gas passes through the lower porous plate before contacting the solid precursor. A compressing plate is located above the lower porous plate for pressing the precursor by its weight. This prior art device comprises a more complex structure than embodied in the simple construction of the present invention.
The vapor generator provided by the present invention, is designed to eliminate the poor and erratic delivery rates exhibited by existing prior art designs as well as their inability to provide complete uniform depletion of the precursor material. The novel device of the present invention is designed to operate as a vapor delivery vessel for solid reagent sources. The bubbler device of the present invention by employing dual porous frit elements achieves saturation of the carrier gas with the organometallic vapor without having to bubble the carrier gas through the solid precursor. Avoiding this action prevented the formation of channels in the solid thereby resulting in a uniform delivery of the material until it was depleted as shown in FIG. 7. The new dual frit bubbler design of the present invention (FIG. 3) overcomes the problems of prior art bubblers not employing porous frit elements by avoiding having to bubble through the solid precursor. The new device design according to the invention can in particular be used in a vapor phase epitaxy system comprising an epitaxy reactor for introducing into the latter at least one of the reagents in the gaseous state, ie. solid organometallic compounds that are used in the semiconductor industry, such as TMI, Cp2Mg, CBr4, etc.
An object of the present invention is to provide a vapor delivery device having a simple and economical construction design for delivering a stable/consistent flow at a higher concentration output than possible from existing bubbler designs.
Another object of the present invention is to provide a vapor delivery device design capable of achieving a constant delivery rate until total depletion of the solid organometallic precursor.
According to still another object of the present invention, there is provided a vapor generator design which is capable of achieving rapid responses to changes in temperature and carrier gas flow parameters.
According to a further object of the present invention, there is provided a vapor delivery design capable of achieving a constant flow rate/concentration delivery over a range of concentrations depending upon the operating parameters such as temperature, pressure, nature of the carrier gas and its flow rate.